A four month old child develops cardiac failure and dies three months later. At autopsy, the hea has no obvious congenital defects, but the cardiac chambers are small and covered with thick, white endocardium. Histological sections of the hea demonstrate regular bands of elastic tissue in the thick fibrous endocardium. Which of the following is the most likely diagnosis?

This disease is endocardial fibroelastosis, which is suspected of being related to intrauterine virus infection, possibly with mumps virus. The endocardium thickens secondary to a marked increase in fibrous and elastic tissue, which may extend into the ventricle. The left ventricle is most commonly involved, but other chambers may also be affected. A variety of other findings may also be present, including mural thrombi, flattened trabeculae, and abnormal (usually stenosed) valves. The disease occurs in infantile and adolescent forms. Coxsackievirus myocarditis would produce a flabby myocardium, patchy areas of hemorrhage, and four chamber dilatation. In cardiac amyloidosis , extracellular amyloid deposits would be seen in histological sections. Idiopathic subaoic stenosis produces a hyperophic cardiomyopathy. Ref: Brown D.W., Fulton D.R. (2011). Chapter 83. Congenital Hea Disease in Children and Adolescents. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds),Hurst's The Hea, 13e.